The trend of wireless local area network (WLAN) and mobile communication moves towards the multi-band/multi-modal system integration, for example, the WLAN 802.a, 802.b, 802.g system. The conventional dual-band RF transceiver device usually includes plural independent transceivers, and each independent transceiver is used for the signals with an individual frequency band. In the front end of the RF system, the research in demands aims at integrating RF components operating at different frequency band.
FIG. 1 shows a schematic diagram of a prior-art dual-band RF front end. As shown in FIG. 1, a complete dual-band RF front end 100 includes a bandpass filter (BPF) element 101, a balun element 105, a power amplifier (PA) element 103, and a low-pass filter (LPF) element 107. Each element has two separated components that are included for operating at frequencies 2.4 GHz and 5.0 GHz, respectively. In a highly integrated multi-band/multi-modal RF structure, an RF filter that is switchable between different passbands will play an important role in functional integration and size reduction of RF front-end module.
Several proposals have been made for the implementation of tunable or switchable RF filters. An RF filter with tunable center frequency is usually implemented with a switch to switch among a number of filters, usually referred to as a filter bank. A filter bank includes a plurality of filters that each has a different center frequency. The input end and the output end of each filter are all connected to switches, which are controlled by an external voltage for switching on the filter corresponding to the required operating frequency, and switching off all the other filters. This type of filter allows the user to switch between any passbands, and usually shows good filtering response. However, because each switched passband requires a filter and two switching circuits, this design has the drawbacks of requiring a larger circuit area and more elements. A few developments have been disclosed to address the above drawbacks.
U.S. Pat. No. 4,571,560 disclosed a switched bandpass filter 200 as shown in FIG. 2. The bandpass filter 200 includes a plurality of triple tuned bandpass filter sections 205a-205c coupled in parallel between an input terminal 203 and an output terminal 201. The input terminal 203 is shunted with a pair of series resonant circuit 203a, 203b while the output terminal 201 is shunted with a pair of series resonant circuit 201a and 201b. Each tuned bandpass filter section includes a respective switch in the internal path, as indicated by s1-s3. As shown in FIG. 2, when the switch is switched off, for example switch s1, the filter is not affected. When the switch is on, as switches s2, s3 in FIG. 2, the filter is grounded and the filter is not in effect. This filter reduces the number of required switches. However, as a filter section is required for each passband, this filter still requires a large filter circuitry.
U.S. Pat. No. 5,917,387 disclosed an RF filter, and the RF filter provides a tunable center frequency across a specified frequency by only one or more resonator elements, wherein normalized frequency response of the filter related to the center frequency remains substantially constant over the specified frequency range. FIG. 3 shows a schematic diagram of the filter.
As shown in FIG. 3, the filter includes a varactor diode inside each resonator element of the filter. The varactor diode is an equivalent of a capacitor, whose capacitance depends on the external control voltage. Therefore, the frequency of the resonator element can be changed by changing the external control voltage. This leads to the change of the center frequency of the filter. With this type of varactor diode, a single filter can operate at different passbands. In comparison with the prior design, the above design successfully reduces the circuit area. However, the drawback of this filter is that the tunable range of operating passbands is small.
Another obstacle to overcome for the tunable or switchable filter is the matching between the output/input terminals and a resonator element when the resonance frequency of the resonator element has a drastic change. This is also referred to as external quality factor. The external quality factor also changes when the frequency changes. This causes a major problem as the filter usually fails to have good matching when the two resonance frequencies have a large difference. Japan Patent No. JP2002009573 disclosed a design including a tunable capacitor or a tunable inductor between a resonance circuit and an input terminal or an output terminal to adjust the matching at different frequencies.